* todos.el: Further code cleanup. Changes to commentary. Remove

* todos.el: Further code cleanup.  Changes to commentary.  Remove
autoload cookies.  Handle todo items in Fancy Diary display.
(todos-done-separator): Reinstate length one less than
window-width until bug#2749 is fixed.
(todos-diary-goto-entry): New function, extracted from adapted
definition of diary-goto-entry, which is removed to be added to
diary-lib.el.
(todos-powerset): Rename from todos-powerset-iterative and remove
defalias by this name.  Add doc string and comments.
(todos-modes-set-1, todos-modes-set-2, todos-modes-set-3)
(todos-mode-external-set): Add doc string.